1. Field of the Invention
The present invention relates to an electrophotographic apparatus using an organic single-layer photosensitive drum. More specifically, the invention relates to an electrophotographic apparatus which is capable of forming an image of excellent quality by using a photosensitive drum of a small diameter.
2. Description of Prior Art
Photosensitive materials which are commercially used for electrophotography include selenium photosensitive materials, amorphous silicon (a-Si) photosensitive materials and organic photosensitive materials. Among them, the organic photosensitive materials are widely used for the applications of personal copies and the like from the overall standpoint of sensitivity, cost, etc.
The organic photosensitive materials are, in many cases, of the so-called function separation type, i.e., the laminated type in which a charge generating layer (CGL) and a charge transporting layer (CTL) are laminated one upon the other. There has further been used an organic photosensitive material of the single-layer dispersion type in which the charge generating substance is dispersed in the charge transporting medium.
The size of the electrophotographic apparatus can be effectively decreased by decreasing the volume occupied by the photosensitive drum, i.e., by decreasing the size of the drum. When the diameter of the drum is decreased, however, a complete image is formed by revolving the drum many times; i.e., the steps of uniform charging, exposure to image, developing, transfer, cleaning and discharging necessary for forming the image must be carried out for many times of revolutions of the drum.
It was, however, found that when a complete image is formed by revolving the organic single-layer photosensitive drum many times, the image density decreases stepwisely depending upon the number of revolutions of the drum.
Though the degree of stepwise decrease in the image density is small, a change in the density on a piece of a copy can be clearly discerned even by the naked eye. This change appears considerably clearly on the solid portion of the image, and it has been urged to solve this problem.
When the image-forming cycle is repeated many times for the above-mentioned organic single-layer photosensitive drum and the a-Si photosensitive material, in particular, the surface potential at the developing portion decreases considerably due to dark attenuation resulting in a great decrease in the image density.